Incomplete dominance is an important concept in the study of genetics. It refers to a circumstance in which the two copies of a gene for a particular trait, or alleles, combine so that neither dominates the other. This creates a new phenotype or set of observable characteristics caused by the interaction of genetics and environment. In short, incomplete dominance is when neither gene is fully dominant, and the result is a brand new trait. Codominance is a similar yet different phenomenon. While in incomplete dominance, neither allele is dominant, in codominance, both alleles are. As a result, codominant traits are expressed equally in the phenotype; that is, the result is a combination of the two traits. That differs from incomplete dominance, in which the result is something new, as the examples of incomplete dominance below demonstrate. It's important to note that most observable traits in any living thing are caused by more than one gene. Incomplete dominance is specific to traits that occur on just one gene. However, there are many such traits, and incomplete dominance occurs in every sort of organism that has genes, including plants, animals and even human beings. Incomplete dominance in animals is most widely studied in domestic animals since it's important for their health, appearance, and value. Here are several examples of the effects of incomplete dominance in animals:
The science of genetics began with plants. People have been interbreeding plants for particular traits since we first started farming more than 11,000 years ago. Gregor Mendel, one of the founders of genetic science, began his studies by recording the ways he planted his garden. Whether for food, other uses, or simple beauty, humans have employed genetic selection of plants, including incomplete dominance, throughout our history.
- Incomplete dominance was first recorded in plants. The German scientist Josef Kolreuter bred red and white carnations, expecting to get offspring with the dominant red coloration. Instead, many came up pink! Kolreuter found that neither allele was fully dominant in his flowers and identified the concept of incomplete dominance.
- Four-o-clocks are flowering plants that get their funny name from their inclination to bloom in the late afternoon. Wild four-o-clocks tend to have red flowers, while "pure" four-o-clocks with no coloration genes are white. Mixing the two results in pink flowers, just like Dr. Kolreuter's carnations. Those pink flowers are a result of incomplete dominance. However, mixing the pink flowers results in ¼ red, ¼ white and ½ pink. That 1:2:1 ratio - a quarter like one parent, a quarter like the other, and the remaining half different from either - is common in cases of incomplete dominance.
- Pink snapdragons are a result of incomplete dominance. Cross-pollination between red snapdragons and white snapdragons result in pink when neither the white or the red alleles are dominant.
- The fruit color of eggplants is another example of incomplete dominance. Combining deep purple eggplants with white eggplants results in eggplants of a light violet color.
- Incomplete dominance is a key element of improving crops such as corn. Corn with multiple incompletely dominant traits is generally healthier and provides greater yields than "purer" strains with fewer such traits. Just compare the original plant, teosinte, with a modern ear of corn to see the genetic difference!
Incomplete dominance is rare in humans; we're genetically complex and most of our traits come from multiple genes. However, there are a few examples. Incomplete dominance is just part of what makes our species so complicated and interesting.
- The disease familial hypercholesterolemia (FH) is an example of incomplete dominance. One allele causes liver cells to be generated without cholesterol receptors, while another causes them to be generated normally. The incomplete dominance causes the generation of cells that do not have enough receptors to remove all dangerous cholesterol from the bloodstream.
- Tay-Sachs Disease is an example of incomplete dominance in humans. This neurological disease is caused by an enzyme imbalance and is autosomal recessive; that is, people who actually suffer from the disease have two recessive genes that cause it. However, one or both of their parents may have been carriers who had incompletely dominant genes, causing them to produce one half of the necessary enzyme, which is enough for a normal life.
- When one parent with straight hair and one with curly hair have a child with wavy hair, that's an example of incomplete dominance.
Eye color is often cited as an example of incomplete dominance. In fact, it's a little more complicated than that, but hazel eyes are partially caused by incomplete dominance of multiple genes related to green and brown eye color.
The Surprising Simplicity of GenesGenetics can be an intimidating subject. Many traits come down to complex interactions between lots of different genes. But by learning the basic mechanisms involved, such as incomplete dominance, it becomes far easier to understand.
For more of the fundamental concepts of genetics, let our article on examples of genotypes and phenotypes help you distinguish between those two confusing concepts.
Examples of Genotype & Phenotype: Differences Defined
Genotype and phenotype are two fundamental terms in the science of genetics. The two terms are often used at the same time to describe the same organism, but there is a difference between genotype and phenotype:An organism’s genotype is the set of genes in its DNA responsible for a particular trait. An organism’s phenotype is the physical expression of those genes. For example, two mice that look virtually identical could have different genotypes. But if they have visibly different traits - say, one has white fur and the other has black fur - then they have different phenotypes.
Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. Unlike complete dominance inheritance, one allele does not dominate or mask the other.
Incomplete dominance occurs in the polygenic inheritance of traits such as eye color and skin color. It is a cornerstone in the study of non-Mendelian genetics.
Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele.
Incomplete genetic dominance is similar to but different from co-dominance. Whereas incomplete dominance is a blending of traits, in co-dominance an additional phenotype is produced and both alleles are expressed completely.
The best example of co-dominance is AB blood type inheritance. Blood type is determined by multiple alleles recognized as A, B, or O and in blood type AB, both phenotypes are fully expressed.
Scientists have noted the blending of traits back into ancient times, although until Mendel, no one used the words "incomplete dominance." In fact, Genetics was not a scientific discipline until the 1800s when Viennese scientist and friar Gregor Mendel (1822–1884) began his studies.
Bettmann Archive / Getty ImagesLike many others, Mendel focused on plants and, in particular, the pea plant. He helped define genetic dominance when he noticed that the plants had either purple or white flowers. No peas had lavender colors as one might suspect.
Up to that time, scientists believed that physical traits in a child would always be a blend of the traits of the parents. Mendel proved that in some cases, the offspring can inherit different traits separately. In his pea plants, traits were visible only if an allele was dominant or if both alleles were recessive.
Mendel described a genotype ratio of 1:2:1 and a phenotype ratio of 3:1. Both would be consequential for further research.
While Mendel's work laid the foundation, it was German botanist Carl Correns (1864–1933) who is credited with the actual discovery of incomplete dominance. In the early 1900s, Correns conducted similar research on four o'clock plants.
In his work, Correns observed a blend of colors in flower petals. This led him to the conclusion that the 1:2:1 genotype ratio prevailed and that each genotype had its own phenotype. In turn, this allowed the heterozygotes to display both alleles rather than a dominant one, as Mendel had found.
As an example, incomplete dominance is seen in cross-pollination experiments between red and white snapdragon plants. In this monohybrid cross, the allele that produces the red color (R) is not completely expressed over the allele that produces the white color (r). The resulting offspring are all pink.
The genotypes are: Red (RR) X White (rr) = Pink (Rr).
- When the first filial (F1) generation consisting of all pink plants is allowed to cross-pollinate, the resulting plants (F2 generation) consist of all three phenotypes [1/4 Red (RR): 1/2 Pink (Rr): 1/4 White (rr)]. The phenotypic ratio is 1:2:1.
- When the F1 generation is allowed to cross-pollinate with true breeding red plants, the resulting F2 plants consist of red and pink phenotypes [1/2 Red (RR): 1/2 Pink (Rr)]. The phenotypic ratio is 1:1.
- When the F1 generation is allowed to cross-pollinate with true breeding white plants, the resulting F2 plants consist of white and pink phenotypes [1/2 White (rr): 1/2 Pink (Rr)]. The phenotypic ratio is 1:1.
In incomplete dominance, the intermediate trait is the heterozygous genotype. In the case of snapdragon plants, plants with pink flowers are heterozygous with the (Rr) genotype. The red and white flowering plants are both homozygous for plant color with genotypes of (RR) red and (rr) white.
Polygenic traits, such as height, weight, eye color, and skin color, are determined by more than one gene and by interactions among several alleles. The genes contributing to these traits equally influence the phenotype and the alleles for these genes are found on different chromosomes.
The alleles have an additive effect on the phenotype resulting in varying degrees of phenotypic expression. Individuals may express varying degrees of a dominant phenotype, recessive phenotype, or intermediate phenotype.
- Those that inherit more dominant alleles will have a greater expression of the dominant phenotype.
- Those that inherit more recessive alleles will have a greater expression of the recessive phenotype.
- Those that inherit various combinations of dominant and recessive alleles will express the intermediate phenotype to varying degrees.